Optical communication systems employing wavelength division mulitiplexing (WDM) use a single fibre to carry multiple traffic channels within a predetermined wavelength band. WDM systems are being deployed increasingly to optimise the transmission capacity of existing networks. Conventional WDM systems typically operate in a wavelength window centred around 1550 nm. They also typically use at least one service channel for carrying system information, for example network control information and control signals. This channel is often carried on the same optical fibre as the traffic information, but outside the traffic carrying bandwidth. For example, a service channel commonly used with the 1550 waveband is carried on the 1510 nm wavelength.
Many optical systems in use today operate in the 1300 nm window. An example is the SONET/SDH. While WDM systems have obvious advantages, their introduction involves considerable investment as new equipment must be installed and maintained even in existing networks. The process is therefore a gradual one. When introducing WDM traffic to existing networks, operators are often obliged to maintain the previous service, at least for a time. The simultaneous operation of a WDM system operating in the 1550 nm window with other services centred round the 1300 nm waveband on the same network, and within the same nodes, requires certain measures to prevent interference between the two services. This includes the installation of filter components, which may comprise simple fused couplers or thin film filters for separating the 1300 nm waveband from the 1550 nm waveband. In non-amplified WDM systems, filter components are necessary for separating out the service channel at each node. However, each filter component causes a basic power loss to channels passively relayed through the filter. The additional 1550 nm/1300 nm couplers along any single link will naturally increase the total link loss, in some cases to a level that exceeds the allowable link loss, such that the WDM system cannot be utilised over the whole network.
There is consequently a need for an arrangement that can allow WDM systems operating in a first wavelength band to be used on existing optical networks at the same time as optical systems operating in a second, different wavelength band.